Patients are placed in a state of general anesthesia for their comfort during surgery. General anesthesia is a reversible state where a patient achieves three conditions: adequate analgesia (insensitivity to pain), a degree of paralysis, and a degree of amnesia. Because these conditions vary from patient to patient, continual observation is necessary to assure a proper state of anesthesia. Furthermore, for each phase of a surgery, the depth of these three conditions varies.
Anesthesia is induced by administering pharmacological agents which include volatile inhalation agents, analgesics, narcotics, and muscle relaxants. Individual patients exhibit a wide range in their physiological response to these drugs, so that anesthesiologists cannot rely on knowledge of the dosage alone to predict the desired state of anesthesia. Furthermore, the effects of these agents vary in time as the patient's body absorbs and metabolizes the drugs. Mixtures of agents can have anesthetic effects which are different than the effects of each agent alone. As mentioned earlier, the conditions of anesthesia may change during each phase of surgery, including initial induction, maintenance during surgery, and recovery. During these times, the anesthesiologist must titrate the patient to the desired state by varying the mixture and dose of the anesthetic agents. Anesthesiologists rely on a variety of observations to assess the adequacy of the state of anesthesia. These observations include examination of the eye and skin, reflex responses to stimuli, change (or cessation) of breathing, and physiological vital signs.
Two commonly observed vital signs are heart rate (HR) and blood pressure (BP). Physiological monitors measure these signs (continually or intermittently) and display their values as single numbers. These values are often displayed on separate instruments. Patients under stress, i.e. under painful stimuli, display normal physiological reactions, such as stimulations of sympathetic and para-sympathetic nervous systems. Notably, they can experience cardiovascular stimulation which causes increases in HR and BP. Thus, anesthesiologists watch HR and BP to determine if they have provided adequate anesthesia. Falling HR and BP can mean too much anesthesia or narcotic, and also indicate generally compromised cardiovascular function.
If the state of anesthesia is not deep enough, even unconscious patients will experience pain and show increases in their HR and BP. Inadequately anesthetized patients can also move involuntarily, which is undesirable during surgery. A state of anesthesia that is too deep may compromise patient organ systems and complicate recovery from the pharmacological agents. Some anesthetic agents have increased toxicity when given in large doses. Therefore, anesthesiologists try to maintain anesthesia that is adequate, but not excessive. This is especially important in patients who are very weak or ill.
Currently, anesthesiologists note the initial "resting" state of the HR and BP before inducing anesthesia. During anesthesia, they observe changes in HR and BP from numbers on displays of two physiological monitors to help judge the state of the patient. Because vital sign displays are read in an area often crowded with other displays, performing the mental calculations about the significance of any physiological changes is subject to some error.
What is needed is a method of display which indicates the amount and degree of change in HR and BP in a single, unambiguous, and recognizable format.